This invention relates to a video projector, also known as a projection television system, using optical modulation means.
The configuration of an optical system of a prior art liquid crystal projector is shown in FIG. 7, wherein three liquid crystal panels of the three primary colors--red (R), green (G), and blue (B)--are used as optical modulation means.
Referring to FIG. 7, the projector is provided with a light source 1 comprising a lamp 10 and a reflective mirror 11. A white-light lamp, such as a metal halide lamp, a xenon lamp, or a halogen lamp is used as the lamp 10. The reflective surface of the mirror 11 is basically parabolic, and the lamp 10 is so disposed that its luminiferous center is at the about the focal position of the parabolic surface. The reflective mirror 11 therefore serves as to collimate the light and the light source 1 therefore emits a substantially collimated beam 100 of white light.
A first dichroic mirror 21 reflects only the red component of white light from the lamp 10, while permitting passage of green and blue components of the light. A reflective mirror 24 reflects the red component of light from the dichroic mirror 21 and supplies it to a liquid crystal panel 30 for red. A second dichroic mirror 22 reflects only the green component of the light (consisting of green and blue components) that has passed through the first dichroic mirror 21 and supplies the green component to a liquid crystal panel 31 for green, while permitting passage of the blue component of light. Reflective mirrors 23 and 25 reflect the blue component of light and supplies it to a liquid crystal panel 32 for blue.
The red, green and blue components of light supplied to the liquid crystal panels 30, 31 and 32 are in the form of beams 100R, 100G and 100B, and are modulated by the liquid crystal panels to form red, green and blue monochromatic image beams. That is, the intensities of each part of the red, green, and blue beams 100R, 100G and 100B are varied by the respective pixels of the liquid crystal panels 30, 31, and 32 that display monochromatic images corresponding to respective color component signals, i.e., an R signal, a G signal, and a B signal, decoded from the video signal that are supplied to the liquid crystal panels 30, 31 and 32, and the liquid crystal panels emit the optically-modulated image beams of the respective colors.
A third dichroic mirror 40, formed of two dichroic mirror components assembled into a cross shape as illustrated, combines the image beams from the liquid crystal panels 30, 31 and 32 into a single beam of full Color (consisting of red, green and blue components). A projection lens 50 magnifies and projects the combined image beam onto a screen 60.
The above prior art liquid crystal projector is associated with the following problems.
A first problem is a relatively complicated optical system is required, the light sources need to emit white light of a relatively high power, and the spatial density of the liquid crystal panels must be high, in order to produce a satisfactory picture quality. The inventors have found that this is due to the fact that the prior art system does not utilize the characteristics of the human sense of sight which has a high resolution with respect to luminance but a lower resolution with respect to color.
Another problem is that it is necessary to accurately position the three liquid crystal panels for red, green, and blue for convergence for each pixel is necessary, in order to prevent color misregistration of the three primary colors. This means that a great deal of time is necessary for adjustment in the assembly process.
Another problem is that, since the third dichroic mirror serving as the image combining means combines the three modulated monochromatic beams is interposed between the liquid crystal panels and the projection lens, the distance between the liquid crystal panels and the projection lens is lone and thus the projection distance is increased. In other words, the projection screen size is increased, but the projected image is likely to be dim.
A further problem is that the red, green, and blue projected images must be registered for each pixel over the entire surface of the screen, requiring complete chromatic aberration compensation at the projection lens.
A further problem concerns a deterioration in the white balance at a center portion of the projected image caused by differences in length of the optical paths from the light source lamp 10 up to the image combining means, and, if this is compensated for, it is likely that this could then lead to deterioration of gray scale resolution in the projected image. This is described below. For example, if the dichroic mirrors 21 and 22 and the reflective mirror 23 are set at the same spacing, the lengths of the optical paths of the red beam 100R and green beam 100G up to the center portion of the third dichroic mirror 40 will be substantially the same, but the length of the optical path of the blue beam 100B up to the same center portion of the third dichroic mirror 40 is longer by the spacing between the first dichroic mirror 21 and the reflective mirror 23. Thus, the illuminance distribution on the liquid crystal panel for blue 32 will be different.
The reason is as follows. Because of the presence of factors such as the arc length of the lamp 10, the light beam emitted from the light source 1 is not completely collimated even if, for example, the collimating means 11 is used. A difference in the illuminance between the central and peripheral parts is greater with regard to the liquid crystal panel for blue supplied with the light beam traveling the longest optical path. This can be compensated for by application of white balance correction signals.
The white balance correction is achieved, by reducing the brightness of the liquid crystal panels which receive stronger light beams. As a result, application of the white balance correction will lowers the gray scale resolution of the picture produced by the projection.